1. Field of the Invention
The present invention relates to semiconductor devices and fabrication methods thereof and, more particularly, to semiconductor devices having a contact window providing low contact resistance and high reliability and fabrication methods thereof.
2. Description of the Related Art
Multilevel interconnection in a semiconductor device is important for achieving higher device density and performance. Especially, structures of contact windows and fabrication method for forming the contact windows are essential for forming the multilevel interconnection. The contact windows electrically connect a wiring layer to a semiconductor substrate or connect a wiring layer to another wiring layer. With increased device density, a depth of the contact window tends to be deeper and its width tends to be narrower. The narrow width of the contact window leads to problems such as a small contact area and, as a result, high contact resistance.
With increased device density, a space between conductive patterns also tends to be closer and closer. This tendency makes the width of the contact window more decreased. This is because the contact window is generally formed to pass through a dielectric material between the conductive patterns. This will be further described below with reference to drawings.
FIGS. 1 to 4 are cross sectional views illustrating a process for forming a contact window in a semiconductor device in accordance with a Korean Patent Laid-open Publication No.99-46930.
Referring to FIGS. 1 and 2, a lower dielectric layer 5, conductive patterns 7 and an upper dielectric layer 9 are formed in sequence on a semiconductor substrate 1 having an impurity active region 3. The impurity active region 3 has a predetermined width 13. The lower dielectric layer 5 has a higher wet etch rate than a wet etch rate of the upper dielectric layer 9 for a selected oxide etching solution such as hydrofluoric acid (HF) solution. The lower dielectric layer 5 is formed of a borophosphosilicate glass (BPSG) layer or a spin-on-glass (SOG) layer. The upper dielectric layer 9 is formed of an undoped silicate glass (USG) layer or a high density plasma (HDP) oxide layer. On the upper dielectric layer 9, a photoresist pattern 11 having an opening is formed to define a contact window area. The upper dielectric layer 9 and the lower dielectric layer 5 are partially removed by a dry etching process using the photoresist pattern 11 as an etch mask. As a result, a contact window 19 is formed to expose the impurity active region 3. At this time, the conductive patterns 7 should not be exposed by the contact window 19 as shown in FIG. 2. Therefore, the width 21 of the contact window 19 should be narrower than the spacing 15 between the conductive patterns 7.
The exposed surface 22 of the impurity active region 3 is severely damaged due to the dry etching process during formation of the contact window 19. Accordingly, the etching damage may increase contact resistance and junction leakage current.
Referring to FIG. 3, the resultant structure is dipped into the selected oxide etching solution to form a final contact window 19′. With this wet etching, the upper dielectric layer 9 is etched to a lesser extent of a selected width 20 in a lateral direction; the lower dielectric layer 5 is etched to greater extent in the lateral direction. Consequently, the final contact window 19′ has a wider width 21′ in a lower region than the width in an upper region. Therefore, an exposed surface area of the impurity active region 3 is increased by the wet etching, as compared with an exposed surface area formed immediately after the dry etching. Referring to FIG. 4, the photoresist pattern 11 is removed. Then, a wiring material 24 is formed on the resultant structure to fill the contact window 19′.
In the prior art, the dry etching exposes the semiconductor substrate 1. If the dry etching is overdone, it may cause a surface damage 22 (See FIG. 2) of the impurity active region 3. Therefore, the dry etching should be controlled with high accuracy to prevent the damage. This dry etching damage causes serious problems such as high contact resistance and high junction leakage current in a semiconductor device.
The contact window 19 should not expose the conductive patterns 7 to insure electrical isolation therebetween. Therefore, a width 17 of the opening of the photoresist pattern 11 and a width 21 of the contact window 19 should not be increased greater than a distance 15 between the conductive patterns 7. In other words, the shorter the spacing between the conductive patterns 7 is, the narrower the width of the contact window 19 is, as described above.
The wet etching also should be controlled with high accuracy, because a distance 26 between the wiring material 24 and the conductive patterns 7 needs to be properly maintained. Assume that the wet etching is overdone and the lower dielectric layer 5 is excessively etched in the lateral direction, and, as a result, the contact window 19′ exposes a bottom side of the conductive patterns 7. This can result in an undesirable electrical connection between the wiring material 24 and the conductive patterns 7. Therefore, in the prior art, the width 21′ in a lower region of the final contact window 19′ cannot be greater than the spacing between the conductive patterns 7. Thus, a contact area between the wiring layer and the conductive patterns 7 is limited by the spacing between the conductive patterns 7. Accordingly, it is difficult to reduce contact resistance.
According to the prior art, the dry and the wet etching should be performed very carefully. That is to say, it is very difficult to maintain the high accuracy necessary to avoid above-mentioned problems during the dry and wet etching.